This invention relates to heat-polymerisable compositions comprising cationically-polymerisable epoxides, aromatic sulphoxonium salts, and organic oxidants, and to the use of these compositions as laminating resins, surface coatings, adhesives, and the like.
British Patent No. 1,516,511 discloses that curable compositions comprising a mono-1,2-epoxide, an epoxide resin (i.e., a substance containing on average more than one 1,2-epoxide group), or a polymerisable mixture thereof, may be polymerised or cured by means of a radiation-sensitive aromatic onium salt of oxygen, sulphur, selenium, or tellurium present in an amount capable of effecting the polymerisation or cure of the epoxide (or polyepoxide) by release of a Lewis Acid catalyst when exposed to radiant energy. The only such salts described in the Specification are of the formula ##STR1## where
R denotes a monovalent aromatic radical,
R.sup.1 denotes an alkyl, cycloalkyl, or substituted alkyl group,
R.sup.2 denotes a polyvalent aliphatic or aromatic radical forming a heterocyclic or fused ring structure,
X denotes oxygen, sulphur, selenium, or tellurium,
M denotes an atom of a metal or a metalloid, such as antimony, iron, tin, bismuth, aluminium, gallium, indium, titanium, zirconium, scandium, vanadium, chromium, manganese, boron, phosphorus, or arsenic,
Q denotes a halogen radical,
a denotes 0, 1, 2, or 3,
b denotes 0, 1, or 2,
c denotes 0or 1, the sum of (a+b+c) being 3 or the valency of X,
d equals (e-f),
f is the valency of M, and is an integer of from 2 to 7, and
e is more than f and is an integer of up to 8.
The Specification also discloses that polymerisation or cure of the composition can be achieved by activating the onium salt to provide release of the Lewis Acid catalyst and that such activation may be achieved by heating the composition to a temperature in the range of from 150.degree. C. to 250.degree. C. (as well as by exposing it to irradiation).
In a subsequent PCT application (WO 80/01723) the same patentee described heat-curable compositions comprising a cationically polymerisable organic material, such as an epoxide resin, and a mixture of a dialkyl hydroxyaryl sulphonium salt and an organic oxidant, which latter term was used to include organic peroxides, iodoso aromatic compounds, and quinones, but also azo-bis(alkyl nitriles) such as azobis(isobutyronitrile). The dialkyl hydroxyaryl sulphonium salts cited are of the general formula ##STR2## where
R represents an aromatic radical of 6 to 20 carbon atoms, having from 1 to 3 nuclearly bonded hydroxy radicals,
each R.sup.1 represents the same or different alkyl radical of 1 to 8 carbon atoms or an alkylene radical capable of forming a cycloaliphatic or heterocyclic ring,
Y.sup.- is a non-nucleophilic anion, and
a is 1 or 2, being 1 when R.sup.1 denotes an alkylene group and 2 when R.sup.1 denotes an alkyl group.
Typical salts of formula II quoted are dimethyl 4-hydroxy-3,5-dimethoxyphenylsulphonium hexafluoroarsenate, dimethyl 4-hydroxy-3,5-dimethylphenylsulphonium hexafluoroantimonate, dimethyl 4-hydroxynaphth-1-ylsulphonium hexafluorophosphate, and 4-hydroxy-3,5-di-tert.butylphenylthiophenium hexafluoroarsenate.
In another PCT application by the same applicant (WO 80/01695) there were described curable compositions comprising a cationically polymerisable organic material and a curing agent consisting essentially of a dialkyl hydroxyaryl sulphonium salt of general formula II and an organic oxidant selected from iodoso aromatic organic compounds, quinones, and organic peroxides, the last being used in the presence of a decomposition accelerator which was an organic amine or a transition metal compound. The curable compositions were stated to provide flexible or rigid organic resin foams when used with a volatile organic solvent as a result of the exothermic cure.
Unpublished experiments by the present applicants have shown that certain aromatic sulphoxonium salts have no effect on epoxide compounds containing 2,3-epoxypropyl groups when heated with them for prolonged periods. Thus, a composition comprising 2 parts by weight of dimethylphenacylsulphoxonium hexafluorophosphate and 150 parts by weight of the most commonly employed, commercially available epoxide resin (2,2-bis(4-glycidyloxyphenyl)propane) did not gel on heating at 150.degree. C. for 30 hours. In other experiments 4.5 parts, 3 parts, and 4 parts, respectively, by weight of acetanilinodimethylsulphoxonium hexafluorophosphate, p-methylacetanilinodimethylsulphoxonium hexafluorophosphate or of p-chloroacetanilinodimethylsulphoxonium hexafluorophosphate were used: the epoxide resin had not gelled after 30 hours at 120.degree. C., 30 hours at 150.degree. C., and 40 hours at 150.degree. C. In the presence of certain oxidants, however, such compositions cured rapidly. Since aromatic sulphonium salts had been described in the above-mentioned prior art as having a heat-curing or heat-polymerising effect on epoxide resins, whereas the sulphoxonium salts in question appeared to exert no effect on prolonged heating with the epoxide resin, it was not to be expected that, in the presence of certain organic oxidants, these sulphoxonium salts would rapidly bring about heat-polymerisation and curing of the epoxide resin.